Field of the Invention
The present invention relates to a receiver and a received channel estimation method for the same. Particularly, the present invention relates to a receiver that receives transmission signals transmitted in a frequency hopping system (technique), being one of spread spectrum communication systems (techniques).
Conventionally, the spread spectrum (SS) communication system is well known as a communication system. The spread spectrum communication system is roughly classified into a direct sequence (referred to as DS) system and a frequency hopping (referred to as FH) system.
The DS system conducts communications by DS modulating original data with a code sequence called a spread code. This system is designed to improve noise resistant characteristics by spreading the signal spectrum through the DS modulation. A pseudo noise (PN) sequence code is generally used as a code sequence for spreading the signal spectrum.
On the other hand, the FH system is the system in which communications is conducted by switching frequency channels in a pseudo random order and in a predetermined order every constant period.
Where communications are conducted using the FH system, the same hopping pattern is previously set to the receiver and the transmitter.
FIG. 11 is an explanatory diagram of the principle of the FH system.
In the FH system referring to FIG. 11, ten channels, f0 to f9, are prepared as frequency channels. In this hopping pattern, the frequency channel is hopped in the order of f0, f1, f2, . . . , f9, f0, f1, . . . .
For convenience in explanation, the hopping pattern is shown in a simple order of frequency channels f0, f1, f2, . . . , f9, f0, f1, . . . . However, an actual hopping pattern is set in such a way that a frequency channel is randomly hopped for each receiver or transmitter.
This example shows 10 frequency channels. However, 20 to 40 channels are actually prepared.
When communications between the receiver and the transmitter is achieved using the FH system, the receiver hops the received frequency (received channel) in synchronization with the transmission frequency (transmission channel) of a transmission signal transmitted from the transmitter. In order to carry out such an operation, the receiver has to establish the so-called initial synchronization which synchronizes the received frequency with the transmission frequency of the transmitter.
FIG. 12 is an explanatory diagram of an operation of establishing initial synchronization carried out in a conventional FH-system receiver.
Since the hopping pattern of a transmission signal transmitted from a transmitter to a self terminal is previously known, the synchronization between the receiver and the transmitter can be established through one-time reception of the transmission signal.
Therefore, if the transmission frequency of a transmission signal from the transmitter is hopped in the order of f0, f1, f2, . . . , f9, f0, . . . , as shown in FIG. 12, the receiver fixes its received frequency to a given frequency channel, for example, f0, of all frequency channels f0 to f9, thus waiting for signals from the transmitter. When the self-discrimination code is detected in the frequency channel f0, synchronization between the receiver and the transmitter is established.
In the conventional synchronization establishing method, the period during which the received frequency of the receiver agrees with the transmission frequency of the transmitter becomes an initial synchronization establishing time. It is desirable to shorten the initial synchronization establishment time as much as possible.
Let us now consider where the longest time is required for initial synchronization in the conventional synchronization establishing method. For example, it is considered that the receiver becomes a receiving state in the frequency channel f0 at the timing t1, with which the transmitter has completely transmitted the transmission signal of a frequency of f0.
If it is assumed that the time period the transmission signal from the transmitter stays in a frequency channel is t1 and that the changeover time of the frequency channel is negligible, the time required for initial synchronization becomes (frequency channel number×residence time t1 ).
Generally, 20 to 40 frequency channels are prepared in the FH system. If it is assumed that the number of channels is 40 and that the residence time t1 per frequency channel is 10 ms, the longest time for initial synchronization establishment in the worst case is 400 ms (=40×10).
This means that communications between the transmitter and the receiver can be first achieved after a lapse of 400 ms from the beginning of the detection of received waves by the receiver. This results in an unrealistic communication system.
In some of actual systems, each channel residence time t1 is, for example, a long period of time of several 100 ms. Such systems require several seconds for initial synchronization establishment.
As described above, the initial synchronization establishment time in the conventional FH system is determined based on the number of channels and the channel residence time t1 . The conventional FH system has the disadvantage of prolonging the initial synchronization establishment time proportional to the number of channels and to the residence time t1.
Moreover, the receiver may not often receive transmission signals from the transmitter upon the initial synchronization establishment due to environments of the communication path between the receiver and the transmitter. The environments includes, for example, multi-path fading produced when transmission radio waves are reflected by a building and synthesized, or shadowing, interference, or the like produced when transmission radio waves are shielded by geometrical features, buildings, trees, vehicles, or the like.
In such a case, the receiver changes the frequency channel for a received frequency to again try initial synchronization establishment. This leads to more adversely prolonging the initial synchronization establishment time.
For countermeasures, it may be considered that frequency channel estimation is performed by receiving over a broad band of all frequency channels in the FH system at an operation for initial synchronization establishment and then digital processing them by the digital signal processor. However, this approach requires digital signal processing circuits LSI or high-speed A/D converters, that can deal with high-speed digital signal processing, thus resulting in increased costs.
Since broadening the received band leads to widening the thermal noise received frequency band, the thermal noise energy WN(WN=k×T×B, where k is the Boltzmann constant, T is an absolute temperature, and B is a band width) increases. As a result, there is the disadvantage in that the receiving sensitivity of the receiver decreases because Eb/No (where Eb is energy per bit and Nb is noise) decreases.
Moreover, when a multi-path fading exists, the signal strength of a received signal greatly varies locally and over time. There is the problem in that since the signal strength of a received signal depends on frequency, the receiving side cannot accurately estimate a frequency channel being used for transmission even if the signal strength of each frequency channel is merely detected at the operation for initial synchronization establishment.
However, it is known that the signal strength varies independently when two receiving spots are spaced away by about λ/2 (where λis wavelength) in the multi-path fading. Conventionally, the receiving antenna diversity technique is known by noting the above phenomenon.
For that reason, it is considered that the adverse effect due to fading can be reduced even in the operation for initial synchronization establishment, provided that signals received by receiving antennas, respectively disposed at plural receiving spots spaced apart a predetermined distance, are used.
There is a first technique for the receiving antenna diversity (branch configuration method) to perform the feedback control on the received signal quality. In the first technique, a changeover switch selects one of plural receiving antennas to measure received signal power or bit error rate. Then, another receiving antenna is selected when the quality of a received signal is less than a predetermined value. However, this method leads to a large delay for processing. Moreover, when a received signal is not determined, the received signal quality cannot be evaluated.
In the second technique for the receiving antenna diversity (synthesis receiving method), signals received with plural receiving antennas amplify respectively and the frequency-converted received signals are synthesized in the highest ratio. However, this method requires not only high-frequency amplifiers in the number corresponding to the number of the receiving antennas but also complicated control. Moreover, there is the technique called “software antenna”. However, this method also requires complicated control.